Microwave mixer



Feb. 9, 1965 P. c. BUTSON 3,169,224

MICROWAVE MIXER Filed July 20. 1961 SIGNAL SOURCE MIXER SIGNAL SOURCE 9 |4 OUTPUT SIGNAL 5 4C7RYSTAL CRYSTAL4 INVEN'I'DR 278R MKLwN in-say W mlw- TQRNEYS 3 Claims. (or. 325-446) This invention relates to a microwave mixer.

It is known to provide a microwave mixer comprising a waveguide having a crystal connected transversely across it. The operation of a mixer of this form is such that, when two signals which are to be mixed are supplied to the waveguide, the desired electric output signal may be derived from across the crystal. In addition, however, the two signals which are being mixed appear across the crystal and it is, therefore, necessary to derive the output signal by way of an output filter, which is arranged to reject these signals.

The need to provide this output filter may involve two diificulties. Firstly, it increases the output capacitance of the mixer, with a consequent decrease in the bandwidth of the output signal which may be derived from the mixer. Secondly, if the output signal is to have a frequency in excess of say, 1000 megacycles, the design of a suitable output filter becomes difiicult.

One object of the present invention, therefore, is to provide a microwave mixer which may be used without an output filter.

According to the present invention, a microwave mixer comprises a waveguide to which, during operation, the signals to be mixed are supplied, a post arranged transversely across the waveguide, and two crystals which are similar to one another and which are coupled in opposition to the two ends respectively of the post, the arrangement being such that, during operation, the required out put signal, which is substantially free of the signals being mixed, may be derived from the post.

Preferably said waveguide is of rectangular crosssection and said post is arranged with its length parallel to the plane of one of the walls of the waveguide and normal to the plane of another wall of the waveguide.

Microwave apparatus in accordance with the present invention will now be described by way of example with reference to the accompanying drawings in which:

FIG. 1 shows a central cross-section of a mixer which forms part of the apparatus; and

FIG. 2 shows the apparatus in block schematic form.

Referring now to FIG. 1 of the drawings, the mixer comprises a short length of waveguide 1, of rectangular cross-section, which is closed at one end, the waveguide 1 being seen end on in the drawing. Adjacent the open end of the waveguide 1 is a post 2 which is arranged with its length parallel to the plane of one of the smaller walls of the waveguide 1 and normal to the plane of one of the larger walls of the waveguide l. The two ends of the post 2 project through two apertures 3 formed in the two larger walls of the waveguide 1, respectively.

The mixer also includes two similar coaxial crystals 4, the inner conductors 5 of which fit into holes 6 formed in the two ends respectively of the post 2. The two crystals 4 are thus symmetrically disposed relative to the post 2 and are both external to the waveguide 1. The open end of the outer conductor 7 of each crystal 4 is arranged to be adjacent the outer surface of the waveguide 1 so as to encircle the aperture 3 through which the adjacent end of the post 2 projects.

The crystals 4 are held in place by short tubes 8. The tubes 8 are external to the waveguide 1 and are arranged to be coaxial with the apertures 3 through 3,169,224 Fatented Feb. 9, 1965 which the ends of the post 2 project, each of the tubes 8 being of such an internal diameter that the relevant crystal 4 is a sliding fit therein. Each tube 8 is externally threaded and is provided with a cap 9 which contains a resilient pad 10, so that when the cap 9 is screwed home a resilient pressure is applied to the base of the crystal 4. Each of the tubes 8 is spaced from the adjacent external wall of the waveguide 1 by a di-electric shim 11 against which the open end of the outer condoctor of the relevant crystal 4 also bears.

The output from the mixer is derived by way of an output coaxial line 12, the inner conductor of which is electrically connected to the mid-point of the post 2 and the outer conductor of which is connected to the waveguide 1. The coaxial line 12 is thus symmetrically arranged with respect to the post 2 and is taken from the waveguide 1 through one of the smaller walls of the waveguide 1. The coaxial line 12 may be connected to the waveguide 1 and post 2 by way of a connector, so that the coaxial line 12 may be disconnected when necessary.

Referring also to FIG. 2 of the drawings, in operation of the mixer, the two signals which are to be mixed are supplied from signal sources 13 and 14 respectively to the waveguide 1 at its open end, the desired output signal then being derived from the coaxial line 12. The mixer is symmetrical about the coaxial line 12, and in operation it is arranged that the dominant waveguide mode is not symmetrical about the output, and has its electric field normal to the inner conductor of the coaxial line 12, so that very little of the signals to be mixed are supplied to the coaxial line 12. The mixer may, therefore, be used without an output filter.

It is not essential for the post 2 to be of right circular cylindrical shape and, in fact, it may be preferable for its cross-section to be reduced at its ends in the manner shown. The operation of the mixer may be improved by otfsetting the post 2 from the centre of the waveguide 1 so that it is then nearer to one of the smaller walls of the waveguide 1 than to the other. If this is done it is desirable to take the coaxial line 12 from the waveguide '1 through the nearer of the smaller Walls of the waveguide 1.

I claim:

1. Microwave apparatus comprising: two signal sources arranged during operation to supply first and second signals which are to be mixed; a mixer including a waveguide having an input end to which both the first and second signals are supplied; a continuously conductive post positioned transversely across the waveguide; two crystals which are similar to one another and which are coupled in opposition to the two ends respectively of the post; and an output conductor which is connected to the post and which is positioned such that the dominant waveguide mode is not symmetrical about said output conductor; during operation the required output signal, which is substantially free of the signals being mixed, being supplied over said output conductor.

2. Microwave apparatus comprising: two signal sources arranged during operation to supply first and second signals which are to be mixed; a mixer including a waveguide of rectangular cross-section having an input end to which both the first and second signals are supplied; a continuously conductive post positioned transversely across the waveguide with its length parallel to the plane of one of the walls of the waveguide and normal to the plane of another wall of the waveguide; two crystals which are similar to one another and which are coupled in opposition to the two ends respectively of the post; and an output conductor which is connected to the post and which is not positioned such that the dominant waveguide mode is symmetrical about said output conductor;

during operation the required output signal, which is substantially free of the signals being mixed, being supplied over said output conductor.

3. Microwave apparatus comprising: two signal sources arranged during operation to supply first and second signals which are to be mixed; a mixer including a waveguide of rectangular cross-section having an input end to which both the first and second signals are supplied; a

post; and an output coaxial transmission line the inner 1 conductor of which extends through an aperture in a wall of the waveguide and is connected to the mid-point of the post; the part of said inner conductor lying within the waveguide being so positioned that the dominant waveguide mode is not symmetrical about said inner conductor; during operation the required output signal, which is sub- 5 stantially free of the signals being mixed, being supplied over said coaxial transmission line.

References Cited by the Examiner UNITED STATES PATENTS 11/51 Rodwin 325--446 2,813,972 11/57 Anderson et a1. 325-435 2,961,531 11/60 Howe 325-445 DAVID G. REDINBAUCH, Primary Examiner. 0 ROY LAKE, Examiner. 

1. MICROWAVE APPARATUS COMPRISING: TWO SIGNAL SOURCES ARRANGED DURING OPERATION TO SUPPLY FIRST AND SECOND SIGNALS WHICH ARE TO BE MIXED; A MIXER INCLUDING A WAVEGUIDE HAVING AN INPUT END TO WHICH BOTH THE FIRST AND SECOND SIGNALS ARE SUPPLIED; A CONTINUOUSLY CONDUCTIVE POST POSITIONED TRANSVERSELY ACROSS THE WAVEGUIDE; TWO CRYSTALS WHICH ARE SIMILAR TO ONE ANOTHER AND WHICH ARE COUPLED IN OPPOSITION TO THE TWO ENDS RESPECTIVELY OF THE POST; AND AN OUTPUT CONDUCTOR WHICH IS CONNECTED TO THE POST AND WHICH IS POSITIONED SUCH THAT THE DOMINANT WAVEGUIDE MODE IS NOT SYMMETRICAL ABOUT SAID OUTPUT CONDUCTOR; DURING OPERATION THE REQUIRED OUTPUT SIGNAL, WHICH IS SUBSTANTIALLY FREE OF THE SIGNALS BEING MIXED, BEING SUPPLIED OVER SAID OUTPUT CONDUCTOR. 